How about reading ahead, so simplifying the required substitution (i.e. modify the previous line of input), e.g.:

#!/usr/bin/perl

use strict;

my ( $prev, $this ) = ( undef(), undef() );
while( <> ) {
    $this = $_;
    /^>>[^>]/ or mySubst( \$prev );
    print $prev;
    $prev = $this;
}

# finally, process the last line of input which got left over
mySubst( \$prev );
print $prev;

sub mySubst {
    my $sref = shift;
    $$sref =~ s/^>>>/PYTHON_PROMPT/;
}

[download]
Update: Having just seen the previous post: The point of reading ahead is to avoid reading the whole file into an array - also (ahem!) the read-ahead version here is one I took a few minutes to test with suitable input.

One world, one people

In reply to Re: Using a regex to catch interpreters by anonymized user 468275
in thread Using a regex to catch interpreters by eastcoastcoder

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.